I hope they don't use CGI, because it might look really obvious that Mystique is just wearing a body suit or that her CGI appearance wouldn't look similar to the previous films where Mystique had body paint/prosthetic.

Surely they will have to use CGI to turn the body suit into blue scales and create the shapeshifting effect? She can't just be filmed wearing a suit, there is going to be post-production done on it.